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Research Updates: How often do uroliths really recur in cats?

The recurrence of uroliths in cats is presumptively a common clinical situation; however, until now, no study has examined recurrence rates or the approximate time until recurrence for the main types of calculi found in cats. This study was performed using the database of the Minnesota Urolith Center at the University of Minnesota. The authors searched the database for cats that had their first ammonium urate, calcium oxalate, or struvite urolith submitted for analysis in 1998 as well as any subsequent calculi from these patients from one or more recurrences between 1998 and 2003. The authors excluded urolith analyses submitted within a six-month window based on the theory that de novo calcium oxalate and ammonium urate urolith formation requires a minimum of six months. Information recorded included the cat's breed, age at time of urolith removal, and sex; the location within the urinary tract and the composition of the uroliths at each removal; and the time elapsed between calculi submissions.

A total of 4,435 cats were included in the study. Regarding the first uroliths analyzed, 54% of the identified cats (n = 2,393) had calcium oxalate uroliths, 41.1% ( n= 1,821) had magnesium ammonium phosphate uroliths, and < 1% (n = 221) had ammonium urate uroliths. Almost all calculi (95.5%) were removed from the lower urinary tract (bladder or urethra).

Urolith recurrence was determined based on repeat urolith analysis in 5.5% of cats (n = 242) during the six-year study period. The frequency of recurrence was 7.1% (first recurrence), 0.6% (second recurrence), and 0.1% (third recurrence) for calcium oxalate uroliths; 2.7% (first recurrence) and 0.2% (second recurrence) for magnesium ammonium phosphate uroliths; and 13.1% (first recurrence) and 4.1% (second recurrence) for ammonium urate uroliths. Urolith composition at the time of recurrence was the same as the initial uroliths in almost all cases, ranging from 83% to 96% depending on the mineral type. Mean interval to first recurrence was between 1.8 and 2.4 years. Older cats initially diagnosed with calcium oxalate or magnesium ammonium phosphate calculi were more likely to have recurrent uroliths identified than younger cats were.